Documentation improvements for 2.1.1.

- Fix typos and broken links.
- Add documentation for functions introduced in 2.1.1.
- Add copyright header to `FLV.m` and remove `hello.c`.
- Update documentation for `mj_ray` and `mj_contactForce`.
- Delete out-of-date comments in `XMLreference.rst`.
- Add geom:size table to `XMLreference.rst`.

PiperOrigin-RevId: 425318966

doc/overview.rst

@@ -197,8 +197,6 @@ The built-in OpenGL visualizer renders this model as:
 If this model is simulated, the box will fall on the ground. Basic simulation code for the passive dynamics, without
 rendering, is given below.
 
-`hello.c <_static/hello.c>`__:
-
 .. code:: c
 
    #include "mujoco.h"